The invention relates generally to a method and apparatus for projecting acoustic signals. More specifically, the invention relates to a transducer system in which electromechanical transducer elements may be driven individually with either a positive or negative voltage of substantially the same magnitude.
Typically, transducer drive electronics include a power supply feeding an amplifier. The amplifier provides gain for an input signal, with the amplified input signal then being fed to a transducer assembly that includes several electromechanical transducer elements. These elements are electrically connected in parallel but operate mechanically in series, so that the mechanical displacements of the transducer elements are summed. In such as approach, all electromechanical transducer elements expand and contract in unison.
Referring to FIG. 1, a prior art acoustic transducer drive system 10 is shown. System 10 includes a very large, ship-based amplifier section 12 that includes of a number of bridge circuits 14 which gate power from power supply 16 to a transformer 18 primary winding "p". By appropriately controlling bridge circuit switches 20A-20D, properly phased, square-wave pulses are introduced into the secondaries "s" of transformers 18. These transformer outputs are then series summed to synthesize a desired waveform, such as a sinewave. The sythesized waveform is typically further amplified, such as by transformer 22 and is then typically tuned via a large, low frequency, tuning inductor 24. Finally, the waveform is conducted down an underwater drive cable and is presented electrically in parallel to a plurality of electromechanical transducer elements 26. The electromechanical elements convert the electrically synthesized signal to mechanical and acoustic power. The transducer elements are marked in this figure so that arrows go from negative to positive for the dipoles in these elements.
This approach to amplification can lead to amplifiers having efficiencies in excess of 80%. For some applications however, such as low frequency transmissions, miniaturization of the amplifiers is limited by the necessary size of the transformers, which can also be the heaviest component of the system. Miniaturization is also impeded by the necessary size of the low frequency, tuning inductors.
It is also known that a large, circulating reactive load is frequently encountered in the drive cable of such systems between the power amplifiers on a ship and the array of transducer elements below. Such reactive loads compel the use of large and heavy drive cables. Further, cross talk is frequently experienced between the alternating current transducer power signals and other signals in the transducer drive cable.
In the area of electromechanical materials, it is known that some of these materials exhibit a nonlinear expansion/contraction response, so that the materials require a direct current to bias the materials to a substantially flat section of their response. Besides these materials, a new generation of electromechanical materials are being researched. These materials do not require polarization, which has its advantages, however the expansion/contraction response of the materials can be highly non-linear.